This invention relates generally to magnetic resonance imaging, and more particularly the invention relates to imaging a portion of an object which is larger than imaging field of view of an MRI system.
Magnetic resonance imaging (MRI) requires placing an object to be imaged in a static magnetic field, exciting nuclear spins in the object within the magnetic field, and then detecting signals emitted by the excited spins as they precess within the magnetic field. Through the use of magnetic gradients and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions.
The static field is typically established by a solenoid magnet in which the object to be imaged is placed. If the desired the portion of the object to be imaged is larger than the field of view of the system, which could be limited by length of the magnet or the gradient coils, the object must be imaged in sections. A common occurrence of this in magnetic resonance angiography (MRA) using an injection of a contrast agent, in which the diagnostic task is to image the peripheral vasculature.
The same problem, a desired field of view that is larger than that which can be collected at one time, also sometimes arises in X-ray angiography. There, the solution has been a xe2x80x9crunoffsxe2x80x9d study, in which a contrast agent injection is made and images are taken at various positions of the patient table with respect to the imaging system. For example, the patient is aligned so as to image the bifurcation of the aorta. The injection is made and as soon as the bifurcation is imaged, the table is moved to image the next xe2x80x9cstationxe2x80x9d, which is imaged as the contrast agent flows in the body of the patient. The table is moved between image acquisitions so as toxe2x80x9cchasexe2x80x9d the contrast agent bolus down the artery tree.
A similar xe2x80x9cstep and shootxe2x80x9d mode is also used with contrast enhanced 3D MRA. Again, the patient is positioned at a first station, the injection is made, and an image of the first portion of the anatomy is collected. This typically takes on the order of 20-40 seconds. The patient is then moved, and the next station is imaged. The table and patient are stationary during imaging and motion takes place between image acquisitions. The images from the multiple sessions can bexe2x80x9csplicedxe2x80x9d together to form a single image of the large volume.
There are several disadvantages with these approaches. First, the time during which the table is moved is wasted. Second, the contrast image distribution is different at the times the various stations are imaged. This combined with possible changes in patient""s position at the time that each station is imaged, causes the spliced complete image to be imperfect at the station boundaries.
The present invention is directed to MRI data acquisition where table motion is required.
In accordance with the invention, MRI data are acquired for a field of view of an object which is larger than the MRI system field of view by continuously moving the object through the system magnet while the object is repetitively excited and MRI signals are detected.
To accomplish the continuous motion, a table for supporting the object is linearly translatable through the magnetic field. In accordance with a feature of the invention, a plurality of coils are provided which move with the table through the magnetic field. Preferably, the coils overlap to ensure full coverage of the object as it passes through the magnetic field.
The invention and objects and features thereof will be more readily apparent from the following detailed description and dependent claims when taken with the drawings.